How do I access a property in the Preferences in 2.80 with Python?
You can access that Preferences property with the following code:

bpy.context.preferences.edit.keyframe_new_interpolation_type


To use this in a panel:

    row.prop(context.preferences.edit, "keyframe_new_interpolation_type")


The full code (using the UI Panel Simple template found in the Text Editor> Templates menu):

import bpy

class HelloWorldPanel(bpy.types.Panel):
    """Creates a Panel in the Object properties window"""
    bl_label = "Hello World Panel"
    bl_idname = "OBJECT_PT_hello"
    bl_space_type = 'PROPERTIES'
    bl_region_type = 'WINDOW'
    bl_context = "object"

    def draw(self, context):
        layout = self.layout
        row = layout.row()
        row.prop(context.preferences.edit, "keyframe_new_interpolation_type")

def register():
    bpy.utils.register_class(HelloWorldPanel)

def unregister():
    bpy.utils.unregister_class(HelloWorldPanel)

if __name__ == "__main__":
    register()


Which displays like this, in the Object tab of the Properties Editor:



If you need to set this value manually you can see the list of strings this property expects to be set to in the Blender 2.80 Python API.